The invention relates to an apparatus for position finding, a positioner and an arrangement which has a positioner and a position finding device.
In many areas of application, it is necessary to find the position of an object or a distance covered by said object. By way of example, in conveying and automation technology, positions of objects need to be monitored and evaluated. A specific field of use in which the accuracy of the position finding is of particular importance is positioning engineering, in which an object is moved in a targeted and defined manner. Such positioners are used in research and industry, for example. Besides the maximum possible reproducibility and scalability for paths of movement, properties such as insensitivity toward vibration, robustness and compactness of the design play an essential part in practical use.
It is already known practice to perform position finding by optical means using a Michelson interferometer. FIG. 1 shows the design of a dual Michelson interferometer 4 which is designed for position finding using the quadrature detection method. A laser 1 produces laser light of wavelength λ which is routed by means of an optical fibre 2 to the interferometer head 3 of the dual Michelson interferometer 4. The interferometer head 3 has a collimator lens 5, two beam splitters 6-1, 6-2, two reference mirrors 7-1, 7-2 and two detectors 8-1, 8-2. The laser light emerging from the fibre end of the optical fibre 2 is widened into a parallel light beam by means of the collimator lens 5. The parallel light beam passes through the first beam splitter 6-1 and second beam splitter 6-2 and is reflected by a mirror 9. The mirror 9 is located on an object (not shown), the position of which in relation to the x direction (see double arrow) needs to be monitored. The light reflected by the mirror 9 in turn passes through the two beam splitters 6-1 and 6-2, with a portion of the reflected light being reflected onto the detectors 8-1, 8-2 in each case. In this context, a light beam reflected in the beam splitter 6-1, 6-2 to the respective detector 8-1, 8-2 interferes with the light beam reflected by the respective reference mirror 7-1, 7-2. The detectors 8-1, 8-2 sense the intensity of the interference pattern. Each measurement signal oscillates periodically on the basis of the displacement x with a periodicity which is given by half the longitudinal wave λ/2 of the laser light.
If the reference mirrors 7-1, 7-2 are arranged such that the interval between the mirror planes thereof is increments of λ/8+N λ/2, where N=0, 1, 2, 3, . . . , the functions=cos(4πx/λ)  (1)
is obtained for the output signal from the detector 8-1 and the functions=sin(4πx/λ)  (2)
is obtained for the output signal from the detector 8-2.
First of all, consideration will be given for the case (not shown in FIG. 1) in which the interferometer head 3 comprises only one interferometer (e.g. comprising the beam splitter 6-1, the reference mirror 7-1 and the detector 8-1). In this case, only the output signal s is provided. This one output signal s is repeated after λ/2. For many applications, however, it is disadvantageous that determining the displacement path x using this one signal s does not allow determination of the direction of displacement and has an accuracy which varies with x (in the region of the extremes of the cosine function, the displacement path determination is possible with substantially lower accuracy than in the edge regions situated in between).
The method of quadrature detection eliminates these two drawbacks. For this purpose, the signal sQ from the second detector 8-2 is likewise evaluated.
The evaluation of the two signals s and sQ allows both determination of the direction of displacement and constant accuracy for the ascertainment of the displacement path x. The price to be paid for this is that a two-channel measuring arrangement (two Michelson interferometers with two detectors 8-1, 8-2) is required.
Furthermore, the dual Michelson interferometer 4 shown in FIG. 1 cannot be miniaturized. The reason for this is that the beam splitters 6-1, 6-2 and the reference mirrors 7-1, 7-2 need to be provided in a solid and therefore physically relatively large unit in order to avoid losing alignment. The dimensions of the most compact dual Michelson interferometers currently available are typically 10×10×5 cm. Since it is not possible to route the light in the measurement section (i.e. between the mirror 9 and the beam splitters 6-1, 6-2) in an optical fibre (on account of temperature effects and mechanical stress, fluctuations in the optical index of the fibre would occur which are equivalent to changes of length in the measurement section), the interferometer head must always be arranged close by and in a prescribed, defined position relative to the object to be monitored. This means that a large number of applications, e.g. in a relatively small self-contained system, in systems where little space is available or in extreme environments, are not possible, since the interferometer head 3 cannot be arranged outside the self-contained system and the measurement light cannot be routed into the system via an optical fibre.
For these and other reasons, there is a need for the present invention.